1. Field of the Invention
This invention relates to a master holder used in the production of stampers for molding optical recording mediums used to record and reproduce information optically, and to an electroforming method using this master holder.
2. Description of the Related Art
Conventionally, the recording of various kinds of information has been effected by using magnetic materials, such as magnetic tapes or magnetic discs, various types of semiconductor memories or the like. While they provide the advantage of easy writing and reading of information, these magnetic and semiconductor memories have certain problems; for example, they allow rewriting of information too easily and are incapable of high-density recording.
To eliminate these problems, an optical information-recording method using optical recording mediums has been proposed as a means of treating various kinds of information effectively; and, as means to be employed in this method, there have been proposed optical information record carriers, recording/reproduction methods, and recording/reproduction apparatuses. In an optical recording medium, serving as an information recording carrier, the recording or reproduction of information is generally effected by virtue of differences in optical reflectance levels, transmittance levels or the like on the surface of the medium's optical recording layer; such differences are caused by partly volatilizing the optical recording layer, causing changes in the reflectance thereof, or deforming the layer, by means of a laser beam. After information has been written to this optical recording layer, it requires no processing, such as a development processing; it is a so-called DRAW (direct read after write) medium which allows "direct reading after writing". Since this optical recording layer allows high-density recording and, further, additional writing, it is effective as an information recording/storage medium.
An optical recording medium generally in use has a pre-format, such as tracking grooves and/or pre-pits, on the surface of its substrate, which substrate is formed, for example, by compression molding, a 2P-method, or injection molding. No matter how this substrate is formed, a stamper is used to transfer a relief pattern on the order of submicrons onto a plastic material, such as a polycarbonate plastic or a polymethyl methacrylate plastic. Such a stamper has conventionally been produced, for example, by the method disclosed in Japanese Utility Model Laid-Open No. 58-141435 or in Japanese Patent Laid-Open No. 61-284843, or, by the method described in "Outline of Optical Disc Processing Technique No. 5" (Nippon Kogyo Gijutsu Center, Mar. 15, 1985).
An outline of a method of producing stampers will be described in detail with reference to FIGS. 5(A) to 5(E). First, a photoresist layer 8 is formed on the surface of a glass substrate 9 (FIG. 5(A)); then, exposure and development processes are performed on the photoresist layer 8 in a pattern corresponding to the pre-format concerned, which pattern is in the form of tracking grooves, information pits, or the like, thereby obtaining a master 6 having a photoresist pattern 8' on its surface (FIG. 5(B)).
Next, a conductive film 11 is formed on the surface of the master 6 (FIG. 5(C)), and then a metal film 12 is formed on the film 11 by electroforming (FIG. 5(D)). After polishing the surface of the metal film 12, the conductive film 11 and the metal film 12 are separated as a whole from the master 6, whereby a stamper 13 for molding information recording mediums is obtained (FIG. 5(E)).
Concerning the generally used method of producing information-recording-medium molding stampers, which has been described above schematically, the steps of FIGS. 5(C) and 5(D) will be explained in more detail. The conductive film 11 is formed, for example, by vacuum deposition of a metal, or by sputtering; this film may be made of silver or, more commonly, nickel. This conductive film, consisting, e.g., of nickel, is formed to a thickness of 500 to 1000.ANG. on the microscopic photoresist pattern 8', which corresponds to the format concerned, which is in the form of tracking grooves, information pits, or the like.
During the electroforming process of FIG. 5(D), the master 6, on which the conductive film 11 has been formed, is held by a master holder; and the electroforming on the master is effected by means of an electroforming apparatus as shown in FIGS. 6(A) and 6(B). The master 6 is turned at a revolving speed of 20 to 30 rpm in a nickel-sulfamate electroforming solution 7, whereby a nickel film is formed on the master 6, on which the conductive film 11 has previously been formed. This process will be illustrated with reference to FIG. 6(A) and 6(B) which show sectional views of an electroforming apparatus. As shown in FIG. 6(A), electricity is first supplied to the nickel-sulfamate electroforming solution 7, with nickel chips 10 being used as the anode and a dummy plate 14 of a highly conductive material, such as copper, as the cathode; whereby the surface oxide of the nickel chips 10 is removed and, at the same time, the nickel-sulfamate electroforming solution 7 is cleaned electrolytically.
Next, as shown in FIG. 6(B), the master 6, with the conductive film 11 formed thereon, is held by a master holder 15 and turned in the nickel-sulfamate electroforming solution 7 at a revolving speed of 20 to 30 rpm, and, while the master 6 is thus being turned, electricity is supplied to the solution, with the nickel chips 10 being used as the anode and the master 6 as the cathode. By this electroforming process, a nickel film is deposited on the master 6 on which the conductive film 11 has previously been formed.
The master holder 15, which is used for the purpose of holding the master 6, with the conductive film 11 formed thereon, is of two types. In the first type, which is shown in FIG. 4(A), a contact ring 18, which serves to transmit electric current from a power source to the conductive film 11, is formed such that it comes in contact with the outer edge portion of the conductive film 11; in the second type, which is shown in FIG. 4(B), the contact ring 18 is in contact with the inner edge portion of the conductive film 11, with electric current from the electrical power source being supplied to the conductive film 11 through a conductor member 19 and the contact ring. In either case, the contact ring must be made of a material having a high conductivity so as to enable the conductive film 11 to be supplied with electric current; generally, copper or a thin plate of SUS is adopted as the material of the contact ring.
A problem with the above-described conventional master holders is that the copper or the thin plate of stainless steel (for example, SUS, or the like) (both have a high conductivity) is partly exposed to the electroforming solution, with the result that a nickel film is also deposited on and adheres to the outer and inner walls of the contact ring. This leads to the problems described in the next paragraph.
As shown in FIGS. 7(A) and 7(B), the metal (nickel) film 12 deposited on the master 6 by electroforming is in such a close contact with the contact ring, as indicated at 17 in the drawings, that, when the master 6 is being released from the holder, the contact ring cannot be easily detached from the metal film 12, with the result that the metal film 12 and the conductive film 11 are partly separated from the substrate 9, as shown in FIG. 9. Thus, in the subsequent polishing process, in which the metal film deposited on the master by electroforming is polished, those sections where such a separation has occurred are exposed to the intrusion of the polishing liquid, with the result that the microscopic relief pattern of the information-recording-medium molding stamper, which is in the form of tracking grooves, information pits, or the like, is impaired.
According to a conventional method, this problem is coped with by using a master having approximately double the size of the effective portion of the stamper (the portion corresponding to the microscopic relief pattern in the form of tracking grooves, information pits, or the like) so that the polishing-liquid intrusion does not reach this effective portion. The trouble with this arrangement is that the unnecessary master portion has to be removed by trimming in the final step and disposed of. This is uneconomical.
Further, with this method, one contact ring can only be used for a single electroforming, which is disadvantageous in terms efficient use of the contact ring and thus in terms of production cost.
According to another method which has been proposed with a view to prevent the metal film from depositing on and adhering to the contact ring, the contact ring 18 is covered with a non conductor material 3, as shown in FIG. 10. A problem with this method is that, as shown in FIG. 8, cracks 5 are generated in those portions of the conductive film 11 which correspond to the interface between the metal film 12 and the non-conductive material 3. Thus, in the subsequent polishing process, the polishing liquid is allowed to intrude through these cracks 5, impairing the minute pattern on the stamper. This seems to be attributable to the fact that the thickness of that portion of the metal film 12 which is in the vicinity of the non-conductor material 3 is particularly large, and, it is considered that due to the deposition of this thick-walled film portion, the stresses of the metal film are locally concentrated in the conductive film 11, causing cracks to be generated therein.